Hmmm. i presume by "the bit on top of the engin" you mean the inlet manifold
check all the gaskets are ok, is there any sign of a leak anywhere?
how bad a smell is it? as you can still run them when they're running rediculously rich - sorry gem i forgot to mention that
knackered injectors wouldn't create a smell, when they go they either start dripping fuel into the engine or get bunged up and stop spraying alltogether so you can rule them out, but make sure they're seated properly - they only push in so just make sure they wont go in any further